Circuit Description; Receive Circuits - Icom IC-M72 Service Manual

SECTION 4

4-1 RECEIVE CIRCUITS

4-1-1 ANTENNA SWITCH (RF UNIT)
The received signals from the antenna connector are
passed through the antenna switch which toggles the
receive (RX) line and transmit (TX) line.
The received signals from the antenna connector are
passed through the low-pass filter (LPF; L81, L82, C80, C83
−C86, C89, C226) and the antenna switch (D52, D90 and
D353 are OFF).
While transmitting, the voltage on the T5V line is applied to
D52, D90 and D353, and these are ON. Thus the TX line is
connected to the antenna, and RX line is connected to the
GND to prevent transmit signal entering.
While receiving, no voltage is applied to D52, D90 and
D353, and these are OFF. Thus the TX line and the antenna
is disconnected to prevent received signals entering, and
RX line is disconnected from the GND and L90, L91, C90
−C92 compose a two-staged LPF which guides received
signals to the RX circuits.
The received signals are applied to the RF circuits.
4-1-2 RF CIRCUITS (RF UNIT)
The RF circuit amplifies received signals within the frequency
coverage. The received signals are filtered at the bandpass
filter (BPF) and amplified at the RF amplifier.
The received signals from the antenna switch are passed
through the two-staged tunable BPF (D92, D93, L92, L93,
C95, C97−C101) to filter out unwanted signals. The filtered
signals are applied to the RF amplifier (Q90). The amplified
signals are then applied to the 1st mixer (Q150) via another
two-staged tunable BPF (D130, D131, L96, L97, C104,
C109, C110, C112−C116).
• 2ND IF AND DEMODULATOR CIRCUITS
RF UNIT
14
D/A
converter
(RF UNIT; IC190)
13
Demodulated signals
to the AF circuits

CIRCUIT DESCRIPTION

8
Filter
amp.
Limiter
Quadrature
amp.
detector
10 11
9
X2
4-1-3 1st IF CIRCUITS (RF UNIT)
The received signals are converted into the 1st IF signal,
and amplified at the 1st IF circuits.
The amplified received signals from the RF circuits are applied
to the 1st mixer (Q150), and converted into the 21.7 MHz 1st
IF signal by being mixed with the 1st local oscillator (LO)
signals from the VCO (Q21, Q22, D21−D23).
The converted 1st IF signal is passed through two 1st IF
filters (FI150, FI151) to filter out unwanted signals, and
applied to the 1st IF amplifier (Q151). The amplified 1st IF
signal is then applied to the FM IF IC (MAIN UNIT; IC170,
pin 16).
4-1-4 2nd IF AND DEMODULATOR CIRCUITS
(MAIN UNIT)
The 1st IF signal is converted into the 2nd IF signal, and
demodulated.
The 1st IF signal from the 1st IF circuits is applied to the
2nd IF mixer in the FM IF IC (IC170, pin 16) and converted
into the 2nd IF signal by being mixed with the 21.25 MHz
2nd LO signal from the reference oscillator (RF UNIT; IC1,
X1) via LPF (RF UNIT; L1, C8, C9, C12).
The converted 2nd IF signal is output from pin 3, and
passed through the 2nd IF filter (FI170) to suppress
sideband noise. The filtered 2nd IF signal is applied to the
limiter amplifier (IC170, pin 5). The amplified 2nd IF signal
is FM-demodulated at the quadrature detector (IC170,
pins 10, 11) and output from pin 9. The demodulated AF
signals are applied to the AF circuits.
BPF
21.25 MHz
FI170
7 5 3
Noise
detector
Mixer
12 14
R5V
"RSSIV" signal to the CPU (IC360, pin 33)
4 - 1
2NDLO
LPF
2
RF UNIT
FM IF IC
(MAIN UNIT; IC170)
16
1st IF signal from the IF amplifier (RF UNIT; Q151)
"NOISV" signal to the CPU (IC360, pin 32)
PLL IC
17
(IC1)
X1
21.25 MHz